Pre-chamber arrangement for a gas engine and a gas engine

ABSTRACT

A pre-chamber arrangement ( 100 ) for a gas engine ( 1 ), including a pre-chamber body ( 20 ) accommodating a volume ( 30 ); and a supply device ( 50 ) for supplying a gaseous medium ( 52 ) into the pre-chamber volume ( 30 ); wherein the pre-chamber body ( 20 ) has a bottom portion ( 22 ) with channels ( 40 ) for allowing gas to flow between the pre-chamber volume ( 30 ) and a main combustion chamber ( 10 ) of the gas engine ( 1 ). Each channel ( 40 ) extends along a channel axis (C) from an inner opening ( 42 ) facing the pre-chamber volume ( 30 ) to an outer opening ( 44 ) configured to face the main combustion chamber ( 10 ). The bottom portion ( 22 ) has a curved outer surface ( 24 ). The channels ( 40 ) are obliquely arranged in relation to a radius (r) of the bottom portion ( 22 ). The bottom portion has flat surfaces ( 46 ) surrounding the outer openings of the channels.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation patent application of U.S. patent applicationSer. No. 17/282,010, filed Apr. 1, 2021, to be granted as U.S. Pat. No.11,261,778 on Mar. 1, 2022, which is a 35 U.S.C. §§ 371 national phaseconversion of PCT/SE2019/050972, filed Oct. 7, 2019, the contents ofwhich are incorporated herein by reference, which claims priority ofSwedish Patent Application No. 1851243-4 filed Oct. 11, 2018, thecontents of which are incorporated by reference herein. The PCTInternational Application was published in the English language. All ofthe aforementioned patent applications are herein incorporated byreference in their entireties.

TECHNICAL FIELD

The present invention relates to a pre-chamber arrangement for a gasengine. The invention also relates to a method for combustion in apre-chamber arrangement, a gas engine comprising such an arrangement anduse of such a pre-chamber arrangement.

BACKGROUND

Combustion engines operating on gaseous fuels are spark ignited and aretypically used for stationary applications. Such gas engines may be usedfor power generation and may, for example, operate on gas from digestersand sewage systems, natural gas or any other biogas. Heavy-duty gasengines are often lean-burn engines, which means burning of fuel with anexcess of air. This results in a lower temperature in the combustionchamber and thereby lower NOx formation. However, lean burning requireshigher ignition/activation energy and pre-chambers are therefore oftenused. Typically, a fully rich fuel gas is supplied into the pre-chamberduring an intake stroke and this fuel gas is subsequently mixed with alean gas mixture from the main combustion chamber of the cylinder. Thisway, a richer mixture of fuel gas and air is achieved in the pre-chamberand this gas mixture is ignited by a spark plug in the pre-chamber. Theflaming gas and active components then spread into the main combustionchamber of the engine. By using a pre-chamber, less NOx is produced andthe thermal efficiency is increased as higher compression ratios can beused.

However, using a pre-chamber is not uncomplicated. For example, due totime constraints, the mixing of the supplied fuel gas and the lean gasmixture from the main combustion chamber seldom or never becomescompletely homogenous. To facilitate mixing between the supplied fuelgas and the lean gas mixture from the main combustion chamber differentsolutions relating to the openings/passages between the pre-chambervolume and the main combustion chamber. Document US 2003213461 A1, forexample, discloses a pre-chamber structure having a plurality oftransfer passages formed in a cup shaped tip portion. The passages areinclined in a tangential plane to achieve a swirling flow of gasentering the pre-chamber volume from the main combustion engine.

SUMMARY

Despite known solutions in the field, it would be desirable to develop apre-chamber arrangement which overcomes or alleviates at least some ofthe drawbacks of the prior art.

An object of the present invention is thus to achieve an advantageouspre-chamber arrangement, which facilitates mixing between suppliedgaseous medium and gas from the main combustion chamber. Another objectof the present invention is to achieve a pre-chamber arrangement, whichenables a more uniform flow into the pre-chamber volume.

The herein mentioned objects are achieved by a pre-chamber arrangement,a gas engine comprising such a pre-chamber arrangement, a method forcombustion in a pre-chamber arrangement and use of such a pre-chamberarrangement disclosed herein.

Hence, according to an aspect of the present invention, a pre-chamberarrangement for a gas engine is provided. The arrangement comprises: apre-chamber body accommodating a volume extending longitudinally along acentre axis (A); and a supply device for supplying a gaseous medium intothe pre-chamber volume; wherein the pre-chamber body comprises a bottomportion with channels for allowing gas to flow between the pre-chambervolume and a main combustion chamber of the gas engine, wherein eachchannel extends along a channel axis from an inner opening facing thepre-chamber volume to an outer opening configured to face the maincombustion chamber, and wherein the bottom portion comprises a curvedouter surface. The channels are (obliquely) arranged such that thechannel axis does not intersect the centre axis of the pre-chambervolume, and the bottom portion comprises flat surfaces surrounding theouter openings of the channels.

According to another aspect of the invention, a gas engine comprisingsuch a pre-chamber arrangement is provided.

Pre-chambers for lean-burn gas engines are normally filled with agaseous medium consisting of a rich fuel gas during an intake stroke inthe cylinder associated with the pre-chamber. However, a rich fuel gascannot be ignited and therefore has to be mixed to increase the amountof air in the gaseous medium. During a compression stroke, a lean gasmixture in the main combustion chamber is typically pressed into thepre-chamber volume and is mixed with the rich fuel gas in thepre-chamber volume. The gas mixture in the pre-chamber volume willthereby become leaner, but will still be richer than the gas mixture inthe main combustion chamber. It is desired to ignite a gas mixture withan air-fuel ratio lower than a stoichiometric (stoich) mixture in thepre-chamber and it is desired to burn a gas mixture with a large amountof excessive air in the main combustion chamber. The gas mixture in thepre-chamber volume will then be ignited by means of a spark plugproducing a small electric spark. The gas mixture of gaseous medium andlean gas mixture in the pre-chamber volume may also be referred to asfuel charge. In order to enable lean burning in the main combustionchamber, higher ignition energy is required. To achieve this, one maycontrol where and how the fuel gas and the lean gas mixture from themain combustion chamber are mixed, and also how fast the gases aremixed. This may be complicated. By having channels that are obliquelyarranged in relation to a radius of the bottom portion, the gas from themain combustion chamber will enter the pre-chamber volume swirlingaround the center axis of the pre-chamber volume. The swirling gas willfacilitate mixing with the gaseous medium in the pre-chamber volume.However, since the channels are inclined and the bottom portion of thepre-chamber body has a curved outer surface, the outer openings will beinclined and thereby have an oval or elliptical shape. Duringcompression, the gas in the main combustion chamber is flowing with highvelocity towards the bottom portion of the pre-chamber body. Anelliptical outer opening provides an uneven gas distribution/velocityinside the channel. The channels will thereby be used unevenly and thegas flow will not follow the direction of the channel axis. This givespoor flow direction stability into the pre-chamber volume and variesdepending on gas velocity. With the pre-chamber arrangement according tothe invention, the flat surfaces surrounding the outer openings willmake the outer openings perpendicular to the channel axis even thoughthe channel is inclined. The outer openings will thereby be circular.The flat surfaces will affect the flow direction of the gas. The gasfrom the main combustion chamber will thereby flow perpendicularly intothe outer openings, the whole channels will be used evenly and the gasflow will follow the direction of the channel axis. This way, a moreuniform flow of gas through the channels will be achieved independentlyof the gas flow velocity or pressure.

The control of gas entering the pre-chamber volume from the maincombustion chamber may thereby be facilitated and more stable resultsare achieved.

Further objects, advantages and novel features of the present inventionwill become apparent to one skilled in the art from the followingdetails, and also by putting the invention into practice. Whereas theinvention is described below, it should be noted that it is notrestricted to the specific details described. Specialists having accessto the teachings herein will recognise further applications,modifications and incorporations within other fields, which are withinthe scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For fuller understanding of the present invention and further objectsand advantages of it, the detailed description set out below should beread together with the accompanying drawings, in which the samereference notations denote similar items in the various drawings, and inwhich:

FIG. 1 schematically illustrates a gas engine according to an example;

FIG. 2 a-b schematically illustrates a pre-chamber arrangement accordingto an example;

FIG. 3 a-b schematically illustrates a pre-chamber arrangement accordingto an example;

FIG. 4 schematically illustrates a pre-chamber arrangement according toan example; and

FIG. 5 illustrates a flow chart for a method according to an example.

DETAILED DESCRIPTION

To achieve a desired combustion in a lean-burn gas engine with apre-chamber and to achieve as clean exhaust gas as possible, a desiredgas mixture should be ignited in the pre-chamber volume. Achieving adesired gas mixture each time by mixing a supplied fuel gas with a leangas mixture from the main combustion chamber, may be very difficult. Toachieve a desired combustion in a cost efficient way and with anacceptable NOx emission level, a pre-chamber arrangement according tothe disclosure has been developed. The present disclosure is applicableon all sorts of gas engines. The disclosure may relate to stationary gasengines, transport gas engines etc. Specifically, the present disclosuremay relate to lean-burn gas engines for power generation.

Hence, according to an aspect of the present disclosure, a pre-chamberarrangement for a gas engine is provided. The arrangement comprises: apre-chamber body accommodating a volume; and a supply device forsupplying a gaseous medium into the pre-chamber volume; wherein thepre-chamber body comprises a bottom portion with channels for allowinggas to flow between the pre-chamber volume and a main combustion chamberof the gas engine, wherein each channel extends along a channel axisfrom an inner opening facing the pre-chamber volume to an outer openingconfigured to face the main combustion chamber and wherein the bottomportion comprises a curved outer surface. The channels are obliquelyarranged in relation to a radius of the bottom portion, and the bottomportion comprises flat surfaces surrounding the outer openings of thechannels. The radius referred to being a radius extending from where thechannel axis intersects the outer opening of the channel to the centrepoint of the bottom portion.

The bottom portion of the pre-chamber body may have an essentiallyhemispherical shape. Alternatively, the bottom portion may have anessentially cylindrical shape. In both examples, the bottom portion hasa curved outer surface through which the channels extend. The flatsurfaces surrounding the outer openings may thus be formed in the curvedouter surface. Without the flat surfaces surrounding the outer openingsaccording to the present disclosure, the outer openings would be formedin a curved surface. Thus, without the flat surfaces surrounding theouter openings, the outer openings would be inclined and elliptical inshape. The curved outer surface may be referred to as convex, arched,curved or similar.

The flat surfaces may be configured, such that they extendperpendicularly to the channel axis of the corresponding channel. Thisway, the gas in the main combustion chamber will enter the channelperpendicularly. Since the bottom portion has a curved outer surfacefacing the main combustion chamber, the flat surfaces will beessentially circular in shape. Also, the flat surfaces will beeccentrically arranged in relation to the channels. Thus, the flatsurfaces are not symmetrically arranged around the outer openings.

It is to be understood that the basic functions of a gas engine and apre-chamber for spark plug ignition are considered to be known and willnot be described in detail herein.

The gas from the main combustion chamber may be a lean gas mixturecomprising a large amount of excess of air. The pre-chamber arrangementmay thus be used for a lean-burn gas engine.

The pre-chamber volume may extend in a longitudinal direction between atop end and a bottom end, wherein the pre-chamber volume is configuredto accommodate an end of a spark plug at the top end. The bottom end ofthe pre-chamber volume is associated with the bottom portion of thepre-chamber body. The pre-chamber volume extends longitudinally along acentre axis or centreline of the pre-chamber volume. The bottom portioncomprises a centre point, from which the radius of the bottom portionextends. The centre axis of the pre-chamber volume extends through thiscentre point. The channels may thus be obliquely arranged in relation tothe centre axis of the pre-chamber volume. In other words, the channelsmay be obliquely arranged in relation to a tangential plane of thecurved outer surface. This means that the channels may be arranged, suchthat the channel axis of each channel does not intersect the centrepoint or the centre axis. The channels may be arranged with the channelaxis extending at an angle in relation to the radius of the bottomportion, wherein the angle is larger than zero. The channels may bearranged with the channel axis extending at an angle in the range of5-15 degrees in relation to the radius of the bottom portion. Thus, thechannels may be arranged with the channel axis being offset in relationto the centre axis/centre point.

The channels in the bottom portion of the pre-chamber body may bereferred to as nozzles. According to an example, the bottom portion ofthe pre-chamber body comprises at least six channels. It is, however, tobe understood that the bottom portion may comprise any number ofchannels. The channels are suitably evenly distributed around thecircumference of the bottom portion.

According to an example, the inner openings have an oval or ellipticalshape. The inner surface of the bottom portion may be concave and sincethe channels are inclined, the inner openings will have an oval orelliptical shape.

The channels may comprise an inner portion, a main portion and an outerportion, where the inner portion comprises the inner opening and theouter portion comprises the outer opening. According to an example, thechannels have a constant cross-sectional diameter. Thus, the innerportion, the main portion and the outer portion may have a similarcross-sectional diameter. Alternatively, the inner portion and/or theouter portion may have a cross-sectional diameter, which differs fromthe cross-sectional diameter of the main portion of the channel.

The inner portion and/or the outer portion may be funnel-shaped.According to an example, the outer openings comprises a rounding or achamfer. The outer portion of the channels may thus comprise a roundingor a chamfer. The outer openings may thereby converge inwards indirection towards the inner opening. By having a rounding or chamfer,the flow coefficient through the channels may be improved.

The channels may extend obliquely downwards from the pre-chamber volumetowards the main combustion chamber.

According to an example, the pre-chamber arrangement further comprises avalve device arranged upstream of the supply device, wherein the valvedevice is configured to open and close depending on the pressure insidethe pre-chamber body. The valve device may thus be configured to controlthe supply of gaseous medium into the pre-chamber volume. The valvedevice may comprise a check valve. The valve device may thus be apassive valve. A passive valve does not need precise control and therebyfacilitates the supply of gaseous medium. The feed pressure of thegaseous medium is the pressure, with which the gaseous medium issupplied into the pre-chamber volume. The feed pressure of the gaseousmedium depends on the engine intake pressure and may be around 0.3 barover the engine intake pressure. At maximum load on the engine, the feedpressure of the gaseous medium may be around 3 bar. During an intakestroke, the pressure in the main combustion chamber and the pre-chamberis lower than the feed pressure of the gaseous medium. The valve devicewill thereby open and gaseous medium will be supplied to the pre-chambervolume via the supply device. During a compression stroke, the cylinderpiston moves upwards towards the pre-chamber volume and presses the leangas mixture into the pre-chamber volume with a pressure typically in therange of 20-40 bar. The pressure in the pre-chamber volume will therebybecome higher than the feed pressure of the gaseous medium and the valvedevice will close. The supply of gaseous medium is thereby terminated.The valve device may thus be configured, such that it is open when thepressure inside the pre-chamber volume is lower than the feed pressureof the gaseous medium, and closed when the pressure inside thepre-chamber volume is higher than the feed pressure of the gaseousmedium.

The pre-chamber arrangement may further comprise a supply channelconfigured to be arranged in fluid communication with a gaseous mediumsource and the supply device. The supply channel may be arranged influid communication with the supply device, via the valve device.

The gaseous medium supplied directly into the pre-chamber volume bymeans of the supply device may be a pre-mixed fuel-air mixture.According to an example, the gaseous medium is a fuel-air mixture withan air-fuel ratio of 0.7. Alternatively, the gaseous medium supplieddirectly into the pre-chamber volume is a fully rich fuel gas, which isnot ignitable.

According to an aspect of the present disclosure a gas engine isprovided, the gas engine comprising at least one cylinder, wherein thegas engine further comprises a pre-chamber arrangement as disclosedherein. The gas engine may be of lean-burn type and the at least onecylinder may comprise an intake for a lean gas mixture. The gas enginemay be configured for power generation. The gas engine may thus beconfigured to drive a generator, generating electrical power.

According to another aspect of the present disclosure, a method forcombustion in a pre-chamber arrangement of a gas engine is provided. Thepre-chamber arrangement comprises: a pre-chamber body accommodating avolume; and a supply device for supplying a gaseous medium into thepre-chamber volume; wherein the pre-chamber body comprises a bottomportion with channels for allowing gas to flow between the pre-chambervolume and a main combustion chamber of the gas engine, wherein eachchannel extends along a channel axis from an inner opening facing thepre-chamber volume to an outer opening configured to face the maincombustion chamber, wherein the bottom portion comprises a curved outersurface, and wherein the channels are obliquely arranged in relation toa radius of the bottom portion, and the bottom portion comprises flatsurfaces surrounding the outer openings of the channels. The methodcomprises: supplying gaseous medium into the pre-chamber volume by meansof the supply device; ensuring that gas from the main combustion chamberenters the pre-chamber volume via the channels; and igniting a gaseousmixture in the pre-chamber volume. The gaseous mixture being ignited inthe pre-chamber may be a mixture of the supplied gaseous medium and thegas from the main combustion chamber. Alternatively, the ignited gaseousmixture is the supplied gaseous medium being a pre-mixed fuel gasmixture.

Supplying gaseous medium inside the pre-chamber volume may be performedduring an intake stroke. The gas from the main combustion chamber may beentering the pre-chamber volume during a compression stroke. Ignitingthe gaseous medium in the pre-chamber volume may be performed by meansof the spark plug producing a small electric spark.

The present disclosure will now be further illustrated with reference tothe appended figures.

FIG. 1 schematically shows a gas engine 1. Gas engine is considered tobe a conventional term and thus means an engine operating on gaseousfuels. The gas engine 1 may be configured for power generation. The gasengine 1 may comprise at least one cylinder 2 and in this Figure showsan example with four cylinders 2. Each cylinder 2 comprises a maincombustion chamber 10 and a piston 12 movably arranged in the maincombustion chamber 10. The gas engine 1 may also comprise a pre-chamberarrangement 100 arranged in fluid communication with the main combustionchamber 10 of each cylinder 2. The pre-chamber arrangement 100 may bereferred to as a pre-combustion arrangement. The pre-chamber arrangement100 is arranged to ignite a gas mixture, whereby the flaming gas andactive components are spread into the main combustion chamber 10 to helpburning a lean gas mixture in the cylinder main combustion chamber 10.Each cylinder 2 comprises an intake 14 for a lean gas mixture. Thepre-chamber arrangement 100 will be further described in FIGS. 2 a-2 cand FIG. 3 .

FIGS. 2 a-2 b schematically illustrates a pre-chamber arrangement 100according to an example. The pre-chamber arrangement 100 is associatedwith a gas engine 1, and more specifically a gas engine cylinder 2, asdisclosed in FIG. 1 . FIG. 2 a shows the configuration of thepre-chamber arrangement 100 and FIG. 2 b show a cross-section view ofthe pre-chamber arrangement 100 as disclosed in FIG. 2 a . For clarity,the references to features of the pre-chamber arrangement 100 asdisclosed in FIG. 2 a may not be shown in FIG. 2 b . The pre-chamberarrangement 100 comprises: a pre-chamber body 20 accommodating a volume30; and a supply device 50, for supplying a gaseous medium 52 into thevolume 30 of the pre-chamber body 20. According to an example, thepre-chamber arrangement 100 further comprises a valve device 54 arrangedupstream of the supply device 50, wherein the valve device 54 isconfigured to open and close depending on the pressure inside thepre-chamber body 20. The pre-chamber body 20 comprises a bottom portion22 with channels 40 for allowing gas to flow between the pre-chambervolume 30 and the main combustion chamber 10 of the gas engine 1,wherein each channel 40 extends along a channel axis C from an inneropening 42 facing the pre-chamber volume 30, to an outer opening 44configured to face the main combustion chamber 10. The bottom portion 22further comprises flat surfaces 46 surrounding the outer openings 44 ofthe channels 40. The flat surfaces 46 may extend perpendicularly to thechannel axis C of the respective channel 40.

The pre-chamber volume 30 may extend longitudinally along a centre axisA between a top end 32 and a bottom end 34. The pre-chamber volume 30 istypically configured to accommodate an end of a spark plug 60 at the topend 32.

The bottom portion 22 comprises a curved outer surface 24, facing themain combustion chamber 10. In this example, the bottom portion 22 hasan essentially hemispherical shape. The bottom portion 22 also comprisesa concave inner surface 26 facing the pre-chamber volume 30. The inneropenings 42 may have an oval or elliptical shape. The flat surfaces 46may have an essentially circular or elliptical shape. The flat surfaces46 may be eccentrically arranged in relation to the channels 40.

The channels 40 may be arranged, so as to provide a rotational movementof the gas 80 entering the pre-chamber volume 30 from the maincombustion chamber 10. In FIG. 2 b is illustrated that the channels 40are obliquely arranged in relation to a radius r of the hemisphericalbottom portion 22. This way, the gas 80 entering the pre-chamber volume30 from the main combustion chamber 10 will have a rotational movement.The bottom portion 22 comprises a centre point P, from which the radiusr of the hemispherical bottom portion extends. The centre axis A of thepre-chamber volume 30 extends through this centre point P. The channels40 may thus be obliquely arranged in relation to the centre axis A ofthe pre-chamber volume 30. The channels 40 may be arranged with thechannel axis extending at an angle α in the range of 5-15 degrees inrelation to the radius r of the hemispherical bottom portion 22.

FIGS. 3 a-3 b schematically illustrate a pre-chamber arrangement 100according to an example. The pre-chamber arrangement 100 is associatedwith a gas engine 1, and more specifically a gas engine cylinder 2, asdisclosed in FIG. 1 . The pre-chamber arrangement 100 may be configuredas disclosed in FIGS. 2 a-2 b . In this example, the pre-chamberarrangement 100 the outer openings 44 comprises a rounding or a chamfer48. Thus, in this example, the outer openings 44 converge inwards indirection towards the inner opening 42. The outer openings 44 will thushave a larger cross-sectional diameter than the inner openings 42.

FIG. 4 schematically illustrates a pre-chamber arrangement 100 accordingto an example. The pre-chamber arrangement 100 is associated with a gasengine 1, and more specifically a gas engine cylinder 2, as disclosed inFIG. 1 . The pre-chamber arrangement 100 comprises: a pre-chamber body20 accommodating a volume 30; and a supply device 50, for supplying agaseous medium 52 into the volume 30 of the pre-chamber body 20.According to an example, the pre-chamber arrangement 100 furthercomprises a valve device 54 arranged upstream of the supply device 50,wherein the valve device 54 is configured to open and close depending onthe pressure inside the pre-chamber body 20. The pre-chamber body 20comprises a bottom portion 22 with channels 40 for allowing gas to flowbetween the pre-chamber volume 30 and the main combustion chamber 10 ofthe gas engine 1. Each channel 40 extends along a channel axis C from aninner opening 42 facing the pre-chamber volume 30, to an outer opening44 configured to face the main combustion chamber 10, as illustrated inFIG. 2 b and FIG. 3 b . The bottom portion 22 further comprises flatsurfaces 46 surrounding the outer openings 44 of the channels 40. Theflat surfaces 46 may extend perpendicularly to the channel axis C of therespective channel 40.

The pre-chamber volume 30 may extend longitudinally along a centre axisA between a top end 32 and a bottom end 34. The pre-chamber volume 30 istypically configured to accommodate an end of a spark plug 60 at the topend 32. The bottom portion 22 comprises a curved outer surface 24,facing the main combustion chamber 10. In this example, the bottomportion 22 has an essentially cylindrical shape. The bottom portion 22also comprises a concave inner surface 26 facing the pre-chamber volume30 as illustrated in FIG. 2 b and FIG. 3 b . The inner openings 42 mayhave an oval or elliptical shape. The flat surfaces 46 may beeccentrically arranged in relation to the channels 40.

The channels 40 may be arranged, so as to provide a rotational movementof the gas 80 entering the pre-chamber volume 30 from the maincombustion chamber 10. The channels 40 are obliquely arranged inrelation to a radius r of the bottom portion 22. This way, the gas 80entering the pre-chamber volume 30 from the main combustion chamber 10will have a rotational movement. The bottom portion 22 comprises acentre point P, from which the radius r of the bottom portion extends.The centre axis A of the pre-chamber volume 30 extends through thiscentre point P. The channels 40 may thus be obliquely arranged inrelation to the centre axis A of the pre-chamber volume 30. The channels40 may be arranged with the channel axis extending at an angle α in therange of 5-15 degrees in relation to the radius r of the bottom portion22.

FIG. 5 illustrates a flow chart for a method according to an example.The figure shows a flow chart for a method for combustion in apre-chamber arrangement of a gas engine. The method thus relates to thegas engine 1 as disclosed in FIG. 1 and to the pre-chamber arrangement100 as disclosed in FIG. 2 a-2 b , FIG. 3 a-3 b and FIG. 4 . The methodcomprises: supplying s101 gaseous medium 52 inside the pre-chambervolume 30 by means of the supply device 50; ensuring s102 that gas 80from the main combustion chamber 10 enters the pre-chamber volume 30 viathe channels 40; and igniting s103 a gaseous mixture in the pre-chambervolume 30.

The step of supplying s101 gaseous medium 52 inside the pre-chambervolume 30 is performed during an intake stroke. The gas 80 from the maincombustion chamber 10 is entering the pre-chamber volume 30 duringcompression. The step of igniting s103 the gaseous medium 52 in thepre-chamber volume 30 may be performed by means of the spark plug 60producing a small electric spark.

The foregoing description of the preferred embodiments of the presentinvention is provided for illustrative and descriptive purposes. It isnot intended to be exhaustive or to restrict the invention to thevariants described. Many modifications and variations will obviously beapparent to one skilled in the art. The embodiments have been chosen anddescribed in order best to explain the principles of the invention andits practical applications and hence make it possible for specialists tounderstand the invention for various embodiments and with the variousmodifications appropriate to the intended use.

We claim:
 1. A pre-chamber arrangement for a gas engine, the arrangementcomprising: a pre-chamber body accommodating a pre-chamber volumeextending longitudinally along a centre axis, the pre-chamber bodycomprises: a bottom portion comprising a curved outer surface thatextends into a main combustion chamber, and channels in the bottomportion for allowing gas to flow between the pre-chamber volume and themain combustion chamber of the gas engine, each channel extends along arespective channel axis from an inner opening facing the pre-chambervolume to an outer opening of the channel configured to face the maincombustion chamber, wherein the channels are arranged such that therespective channel axis of each channel does not intersect the centreaxis of the pre-chamber volume, and in that the bottom portion comprisesflat surfaces surrounding the outer openings of the channels.
 2. Thearrangement according to claim 1, wherein the bottom portion comprisesat least six channels.
 3. The arrangement according to claim 1, whereinthe channels are arranged with the respective channel axis extending atan angle in a range of 5-15 degrees in relation to a radius of thebottom portion, the radius of the bottom portion extending from wherethe channel axis intersects the outer opening of the channel.
 4. Thearrangement according to claim 1, wherein the inner opening has an ovalor elliptical shape.
 5. The arrangement according to claim 1, whereineach channel has a constant cross-sectional diameter.
 6. The arrangementaccording to claim 1, wherein the outer opening of each channelcomprises a rounding or a chamfer.
 7. The arrangement according to claim1, wherein each channel extends obliquely downwards from the pre-chambervolume towards the main combustion chamber.
 8. A gas engine, comprisingat least one cylinder, wherein the gas engine further comprises apre-chamber arrangement according to claim
 1. 9. A pre-chamberarrangement for a gas engine, the arrangement comprising: a pre-chamberbody accommodating a pre-chamber volume extending longitudinally along acentre axis; a supply device for supplying a gaseous medium into thepre-chamber volume; and a valve device arranged upstream of the supplydevice, wherein the valve device is configured to open and closedepending on a pressure inside the pre-chamber body, wherein thepre-chamber body comprises: a bottom portion comprising a curved outersurface that extends into a main combustion chamber, and channels in thebottom portion for allowing gas to flow between the pre-chamber volumeand the main combustion chamber of the gas engine, each channel extendsalong a respective channel axis from an inner opening facing thepre-chamber volume to an outer opening of the channel configured to facethe main combustion chamber, wherein the channels are arranged such thatthe respective channel axis of each channel does not intersect thecentre axis of the pre-chamber volume, and in that the bottom portioncomprises flat surfaces surrounding the outer openings of the channels.10. The arrangement according to claim 9, wherein the bottom portioncomprises at least six channels.
 11. The arrangement according to claim9, wherein the channels are arranged with the respective channel axisextending at an angle in a range of 5-15 degrees in relation to a radiusof the bottom portion, the radius of the bottom portion extending fromwhere the channel axis intersects the outer opening of the channel. 12.The arrangement according to claim 9, wherein the inner opening has anoval or elliptical shape.
 13. The arrangement according to claim 9,wherein each channel has a constant cross-sectional diameter.
 14. Thearrangement according to claim 9, wherein the outer opening of eachchannel comprises a rounding or a chamfer.
 15. The arrangement accordingto claim 9, wherein each channel extends obliquely downwards from thepre-chamber volume towards the main combustion chamber.
 16. A gasengine, comprising at least one cylinder, wherein the gas engine furthercomprises a pre-chamber arrangement according to claim
 9. 17. A methodfor combustion in a pre-chamber arrangement of a gas engine according toclaim 1 or 9, the method comprising: supplying gaseous medium into thepre-chamber volume by the supply device; ensuring the gas from the maincombustion chamber enters the pre-chamber volume via the channels; andigniting a gaseous mixture of the supplied gaseous medium and the gasfrom the main combustion chamber.
 18. The method according to claim 17,wherein the bottom portion comprises at least six channels.
 19. Themethod according to claim 17, wherein the inner opening has an oval orelliptical shape.
 20. The method according to claim 17, wherein eachchannel extends obliquely downwards from the pre-chamber volume towardsthe main combustion chamber.